Protective coatings are needed to protect a variety of substrates from premature deterioration and failure due to the particular environment in which they are used. Such substrates include concrete, steel and other industrial building or structural materials that are typically used in environments well known for their corrosive, chemical, mechanical, thermal and atmospheric degradation. Such applications include steel structures used in industrial chemical processing plants, oil refineries, power plants, offshore drilling platforms and the like. To be effective in such applications, the protective coating should protect against a variety of conditions. It must be a corrosion barrier; must be weatherable, that is, resistant to ultraviolet light and other components of sunlight as well as environmental constituents; must be heat resistant; and must be chemically resistant.
The performance of a protective coating is greatly dependent on the resin system used as the binder for the composition. Coating binders have historically been chosen from the following categories of resins: epoxy, polyurethane, silicone, silicate, acrylic, vinyl, alkyd, chlorinated rubber and the like. Some of these generic classes require a high level of organic solvent to dissolve them and cannot be used in light of today's raised environmental consciousness. Accordingly, the state-of-the-art in high performance coatings is defined primarily by epoxy, polyurethane, silicone and silicate binders.
Each of these resins are known for their individual unique characteristics. For example, an epoxy resin binder provides the properties of enhanced corrosion and chemical resistance. A polyurethane resin binder provides enhanced weatherability and appearance. A silicone resin binder provides enhanced heat resistance and weatherability. Silicate resin binders, when combined with zinc dust, provide long-lived corrosion resistance. However, each resin of this current class of binders is also characterized as having limited performance potentials in certain areas. Epoxy resin binders tend to cure slowly at temperatures below 10.degree. C. and have poor weatherability properties. Polyurethane resin binders are moisture sensitive, derived from toxicologically hazardous polyisocyanates, and known to retain their appearance for only three to seven years. Silicones require baking or high heat curing to achieve full performance and are marginal film formers.
True advancements in the state-of-the-art for protective coatings require substantial improvements in weathering (primarily ultraviolet resistance), heat resistance, chemical resistance, and corrosion control. Polysiloxane chemistry offers the potential for providing many of these advancements. Polysiloxane is defined as a polymer consisting of repeating silicon-oxygen atoms in the backbone that imparts several advantages over previously used carbon-based polymer binders; one of these advantages being an enhanced chemical and thermal resistance due to the silicon-oxygen bond. Polysiloxane's polymer linkage is also transparent to ultraviolet light making it resistant to ultraviolet degradation. Finally, polysiloxane is not combustible and is resistant to a wide range of chemicals and solvents, including acids.
Exemplary of polysiloxane coating compositions is that described by Law et al. in U.S. Pat. No. 4,113,665. Law discloses a process for making chemically resistant coatings by reacting, in an acid medium, trialkoxysilanes and silicone intermediates. The Law invention represented a major advancement in polysiloxane based coatings technology because it provided a means of providing ambient temperature curing of polysiloxane compositions. Unlike conventional silicone compositions, the Law patent provided a process for achieving the full chemical and heat resistance properties of silicone based materials without the need for high temperature curing.
Although the process disclosed in the Law patent provided the improvement of ambient temperature curing, it has certain inherent limitations. The use of an acid catalyzed reaction requires a prehydrolysis step that necessitates a considerable input of energy for an extended period of time, thereby increasing the manufacturing cost of such products. The acid catalyzed production of polysiloxane produced protective coatings poorer than desired, were limited to either semi-gloss or flat finishes. Such an acid catalyzed composition must be marketed as a two-part product. The use of such a two-part product requires that each part be combined immediately before application. Often it is desirable to have a coating composition in a single container for field applications where mixing of two parts may not be reliable.
It is, therefore, highly desirable to provide a high performance protective coating binder composition affording improved protection from corrosion and attack by chemicals, solvents, weathering, and heat where the binder can be manufactured economically and cured at ambient temperature. It is further desired that the binder produces a protective coating having a high-gloss finish with a relatively low viscosity that can be supplied in a one-package system and applied without a large degree of organic solvent thinning.